What are the dogmatic differences between Jehovah's Witness Churches and Pentecostal Churches? Until recently I thought both churches were one and the same in beliefs.
The short version is that they are not related at all. In fact, in many major ways they are quite opposite to each other.
There are some similarities in the early roots of Pentecostalism. One of several main roots of the movement can be traced through the Foursquare Church. Although that particular church has since taken a bit more moderate approach, its beginnings are tied up in the affairs of one Aimee Semple McPherson and her teachings, "healing ministry," and other extensive influence. Because it has many roots, and later many branches, the Pentecostal movement is not as organized or definable as a sect, but it does have some characteristic teachings such as an emphasis on continued revelation, spiritual gifts, miraculous healings, and so on.
On the other hand, the JW doctrine tends to be much more concrete. They believe in various specific end times predictions (some of which have morphed as dates come and go), and are very precise about things like terms of service to the church, doing good deeds, and so on.
In summary, while they both began at roughly the same time and some parallels could be drawn in the way they first started, they are completely different entities. Early on they were driven by very different influences, and they have become very different beasts. Their teachings bear little resemblance to each other, and many of their teachings are mutually exclusive.
Basically, a lot.
Jehovah's Witnesses are not really related to Pentecostalism
For one thing, Jehovah's Witnesses teach that the Watchtower literature is the most accurate interpretation of scripture, while Pentecostals believe in scripture being relevant to modern life without having to rely on the leadership of a church to lead them; the Jehovah's Witnesses believe in "divine guidance" that allows them to claim that certain parts of scripture are poorly translated or inaccurate.
There are major soteriological differences: Pentecostals basically believe that faith alone is sufficient for salvation, often but not always including baptism in the Holy Spirit, while Jehovah's Witnesses have a doctrine that requires works for salvation (to prove that their faith is genuine).
In addition, Jehovah's Witnesses believe that we are in the end times (making various apocalyptic predictions that haven't seemed to come true), while Pentecostals don't really have a defined statement on that as a larger movement.
The two groups are, to a certain degree, apples and oranges.
Jehovah's Witnesses are a sect, and Pentecostalism is a movement. I attended a Pentecostal church, and they were relatively midline for Pentecostals, but there was no real organization that they were a part of; meanwhile, the Jehovah's Witnesses have a hierarchy and direct leadership. You may find individual Pentecostal movements that have different beliefs, or incorporated beliefs of other sects, but as a general rule they are not related to the Jehovah's Witnesses, who predate them.
Massive differences. On the outside, the Jehovah's Witness organization appears to be a regular Christian church. But JWs hold to a very different set of beliefs. Dig deeper and you'll find that: